336 COOK 



prove that the higher qualities are caused by the environment, 

 but only that they require certain conditions in which to develop 

 and maintain themselves. 



Environment is of the first importance to individual organ- 

 isms, but the inference so widely drawn in scientific and general 

 literature, that the environment causes and controls evolution, is 

 essentially fallacious. It controls, in a measure, by limiting 

 some of the avenues of advance, or by setting higher and 

 higher requirements for continued progress, but life finds mil- 

 lions of different ways to solve its environmental problems. 

 Given a particular environment and a particular selection of 

 individuals with their hereditary qualities and habits known, and 

 we may with confidence expect a fairly definite reaction in line 

 with previous experiments of the same kind. But this does not 

 mean that evolution is an environmental cul de sac. Changes 

 are not passive merely, but kinetic. The environmental possi- 

 bilities are persistently tested by many variations. Species have 

 retained in this way the power of ameboid motion, and have thus 

 crept over the whole face of nature, and into all the crevices. 



The progress possible in a single life-time or generation may 

 be small, but the lesson is plain. The largest, most practical, 

 and most precious factors of amelioration for plants, animals 

 and men, lie in the discovery and preservation of those indi- 

 viduals which are in the line of evolutionary advancement for 

 the breed — those possessing the qualities required by the en- 

 vironment, and which at the same time strengthen the species 

 and help to maintain the necessary vital motion in courses of 

 beneficial change. 



THE PURITY OF GERM-CELLS AND CHROMOSOMES. 



In the search for causes of natural phenomena an important 

 step appears to have been taken when definite quantitative re- 

 lations have been established. It is not strange, therefore, that 

 the discovery of Mendelian or "disjunctive" hybrids should 

 have aroused much interest, and even a certain amount of excite- 

 ment, among biologists. Mathematical considerations have 

 been allowed to obscure biological facts, and Mendel's "prin- 

 ciples of inheritance" have been declared to be as fundamental 



